A new direct method for measuring the Hubble constant from reverberating accretion discs in active galaxies

Abstract

We show how wavelength-dependent time delays between continuum flux variations of active galactic nuclei (AGNs) can be used to test the standard black hole—accretion disc paradigm, by measuring the temperature structure T(R) of the gaseous material surrounding the purported black hole. Reprocessing of high-energy radiation in a steady-state blackbody accretion disc with T ∝ R−3/4 incurs a wavelength-dependent light travel time delay τ∝λ4/3. The International AGN Watch multiwavelength monitoring campaign on NGC 7469 showed optical continuum variations lagging behind those in the UV by about 1 d at 4800 Å and about 2 d at 7500 Å. These UV/optical continuum lags imply a radial temperature structure T ∝ R−3/4, consistent with the classical accretion disc model, and hence strongly support the existence of a disc in this system. We assume that the observed time delays are indeed caused by a classical accretion disc structure, and derive a redshift-independent luminosity distance to NGC 7469. The luminosity distance allows us to estimate a Hubble constant of H0 (cos i/0.7)1/2 = 42 ± 9 km s−1 Mpc−1. The interpretation of the observed time delays and spectral energy distribution in the context of an accretion disc structure requires further validation. At the same time, efforts to minimize the systematic uncertainties in our method to derive a more accurate measurement of H0, e.g. by obtaining an independent accurate determination of the disc inclination i or statistical average of a moderate sample of active galaxies, are required. However, this remains a promising new method of determining redshift-independent distances to AGNs.